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During the 2018 WorldDendro fieldweek in Bhutan, we examined the flood history of the Dhur River. Most villages are located along streams, so knowing the flood history of the area will enable managers to prepare for future events. We collected scarred partial cross sections from 29 trees along a two km stretch of the Dhur River, and two cores per tree from 29 other trees from six species (Populus ciliata, Picea spinulosa, Tsuga dumosa, Quercus semecarpifolia, Pinus wallichiana, and Rhododendron arboreum). We identified large flood events in 2009, 1989, and 1967 from at least two trees with flood scars or traumatic rings. Our flood-scar chronology extends to 1940 with five cross-sectioned trees, and back to 1904 with core evidence from two trees. The oldest flood scar occurred in 1967. The 2009 flood scar was recorded in most of our streamside samples and is the result of heavy precipitation from Cyclone Aila at the end of May 2009. Two other storms and proceeding flood events occurred in 1989 and 1967 according to additional scars detected in several samples. This work demonstrates the successful use of density fluctuations in Pinus along with scarring in multiple species to reconstruct past flood events and identifies the effects of Cyclone Aila, as an extreme event for this area, which was unprecedented for the past hundred years.

We present here the results of spruce (Picea schrenkiana Fish. et May.) tree-ring research in the Tien Shan Mountains, Kirgiz Republic. We explore the connection between climatic parameters and spruce ring width and maximum density at the upper and lower tree limits and provide two reconstructions: the May-August temperature reconstruction from 1626 to 1995 based on a multi-site composite maximum density chronology from the upper tree limit and the drought index reconstruction from 1680 to 2000 based on the lower tree limit regional ring width chronology. The ring width chronologies from the upper and lower tree limits show a strong similarity. They both depend to a large extent on moisture availability. The maximum density chronology does not correlate with them: it depends on different climatic parameters, namely on the summer temperature. The correlations of the reconstructions with CRU TS3 temperature and precipitation grid point data confirm the results of the modeling using the meteorological data from the nearest stations. The 20th century does not look unusual in the context of the last three hundred years in the Tien Shan Mountains, either in terms of the drought occurrence and severity or in summer temperature changes. However the reconstruction does not encompass the last decade when the summer warming in Tine Shan has been especially prominent. In contrast, some change in precipitation is indicated with the 19th century being drier in the Issyk Kul region compared to the 20th century.

The amplitude of El Nino-Southern Oscillation (ENSO) varies substantially at each phase of its evolution, affecting the timing and patterns of atmospheric teleconnections around the globe. Instrumental records are too short to capture the full behavior of ENSO variability. Here we use the well-validated Monsoon Asia Drought Atlas (MADA) and North America Drought Atlas (NADA) for the past 700 years, and show that tree-ring records from different regions represent tropical sea surface temperature (SST) conditions at various phases of ENSO. Three modes of tree-ring based summer drought variability are found to be correlated with ENSO: summer droughts over the Maritime Continent and Southwest North America (NA), and a dipole mode between Central and South Asia. A lagged correlation analysis is performed to determine the time when precipitation and temperature anomaly imprints on summer droughts as recorded in tree-rings. Drought anomalies in the Maritime Continent and Southwest NA represent ENSO at the developing and peak phases respectively, while those over Central/South Asia are associated with tropical-wide SST anomalies (including the Indian Ocean) at the decay phase of ENSO. Thus proxy records from different regions can provide valuable information on long-term behavior of ENSO at different phases.

Proxy-based climate reconstructions can extend instrumental records by hundreds of years, providing a wealth of climate information at high temporal resolution. To date, however, their usefulness for informing climate risk and variability in policy and social applications has been understudied. Here, we apply tree-ring based reconstructions of drought for the last 700 years in a climate index insurance framework to show that additional information from long climate reconstructions significantly improves our understanding of the underlying climate distributions and variability. We further show that this added information can be used to better characterize risk to insurance providers, in many cases providing meaningful reductions in long-term contract costs to farmers in stand-alone policies. The impact of uncertainty on insurance premiums can also be reduced when insurers diversify portfolios, and the availability of long-term climate information from tree rings across a broad geographic range provides an opportunity to characterize spatial correlation in climate risk across geographic regions. Our results are robust to the range of climate variability experienced over the last 400 years and in model simulations of the twenty-first century, even within the context of changing baselines due to low frequency variability and secular climate trends. These results demonstrate the utility of longer-term climate histories in index insurance applications. Furthermore, they make the case from a climate-variability perspective for the continued importance of such approaches to improving the instrumental climate record, even into a non-stationary climate future.

Previous tree-ring studies indicate that tree growth at high elevations is strongly limited by temperatures in the southeastern China, where the climate is dominated by the East Asian monsoon. Based on this result, we built a highly replicated 202-year tree-ring width chronology from high elevation sites in the Dabie Mountains, southeastern China. The most reliable period of the chronology is from 1834 to 2011 according to a subsample signal strength cutoff of 0.85. Based on this chronology, January–July minimum temperature was reconstructed for the last 178 years, with an explained variance of 57.6% during the instrumental period 1956–2010. The reconstructed temperature series matches reasonably well with three other tree-ring based temperature reconstructions at decadal time scales in the region. The coldest periods are 1891–1898 and 1904–1914, however, the longest cold period is from 1948 to 1973. The warmest period is 1990–2010. Both the recent (1990–2010) warming and recent (1948–2010) temperature increase rates are unprecedented compared with any same long period during the past 178 years in the study region.

Past global climate changes had strong regional expression. To elucidate their spatio-temporal pattern, we reconstructed past temperatures for seven continental-scale regions during the past one to two millennia. The most coherent feature in nearly all of the regional temperature reconstructions is a long-term cooling trend, which ended late in the nineteenth century. At multi-decadal to centennial scales, temperature variability shows distinctly different regional patterns, with more similarity within each hemisphere than between them. There were no globally synchronous multi-decadal warm or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age, but all reconstructions show generally cold conditions between AD 1580 and 1880, punctuated in some regions by warm decades during the eighteenth century. The transition to these colder conditions occurred earlier in the Arctic, Europe and Asia than in North America or the Southern Hemisphere regions. Recent warming reversed the long-term cooling; during the period AD 1971?2000, the area-weighted average reconstructed temperature was higher than any other time in nearly 1,400 years. Fil: Ahmed, Moinuddin. Federal Urdu University of Arts, Science and Technology. Department of Botany; Pakistán Fil: Anchukaitis, Kevin J.. Columbia University. Lamont Doherty Earth Observatory; Estados Unidos. Woods Hole Oceanographic Institution; Estados Unidos Fil: Asrat, Asfawossen . Addis Ababa University. School of Earth Sciences; Etiopía Fil: Borgaonkar, Hemant P.. Indian Institute of Tropical Meteorology; India Fil: Braida, Martina . University of Trieste. Dipartimento di Matematica e Geoscienze; Italia Fil: Buckley, Brendan M. . Columbia University. Lamont Doherty Earth Observatory; Estados Unidos Fil: Ulf Büntgen. Swiss Federal Research Institute WSL; Suiza Fil: Chase, Brian M.. Université Montpellier. Département Paléoenvironnements et Paléoclimats (PAL); Francia. University of Bergen. Department of Archaeology, History, Cultural Studies and Religion; Noruega Fil: Christie, Duncan A. . Universidad Austral de Chile. Laboratorio de Dendrocronología y Cambio Global; Chile. Universidad de Chile. Center for Climate and Resilience Research; Chile Fil: Cook, Edward R.. Columbia University. Lamont Doherty Earth Observatory; Argentina Fil: Curran, Mark A. J.. Australian Antarctic Division; Australia. University of Tasmania. Antarctic Climate & Ecosystems Cooperative Research Centre; Australia Fil: Diaz, Henry F. . National Oceanic and Atmospheric Administration. Cooperative Institute for Research in Environmental Sciences, ; Estados Unidos Fil: Esper, Jan. Johannes Gutenberg University. Department of Geography,; Alemania Fil: Fan, Ze-Xin . Chinese Academy of Sciences. Xishuangbanna Tropical Botanical Garden ; China Fil: Gaire, Narayan P. . Nepal Academy of Science and Technology. Faculty of Science,; Nepal Fil: Ge, Quansheng. Chinese Academy of Sciences. Institute of Geographical Sciences and Natural Resources Research, ; China Fil: Gergis, Joëlle. University of Melbourne. School of Earth Sciences; Australia Fil: González-Rouco, J Fidel. Universidad Complutense de Madrid. Departamento Astrofísica y CC de la Atmósfera; España Fil: Goosse, Hugues. Université Catholique de Louvain. LemaitreCenter for Earth and Climate Research, Earth and Life Institute; Bélgica Fil: Grab, Stefan W. . University of the Witwatersrand. School of Geography,Archaeology and Environmental Studies; Sudáfrica Fil: Graham, Nicholas. Hydrologic Research Center; Estados Unidos Fil: Graham, Rochelle . Hydrologic Research Center; Estados Unidos Fil: Grosjean, Martin . University of Bern. Oeschger Centre for Climate Change Research & Institute of Geography; Suiza Fil: Hanhijärvi, Sami T.. University of Helsinki. Department of Environmental Sciences; Finlandia Fil: Kaufman, Darrell S.. Northern Arizona University. School of Earth Sciences and Environmental Sustainability; Estados Unidos Fil: Kiefer, Thorsten . International Project Office, Past Global Changes (PAGES); Suiza Fil: Kimura, Katsuhiko. Fukushima University. Department of Symbiotic System Science; Japón Fil: Korhola, Atte A.. University of Helsinki. Department of Environmental Sciences; Finlandia Fil: Krusic, Paul J.. Stockholm University. Department of Physical Geography and Quaternary Geology; Suecia Fil: Lara, Antonio. Universidad Austral de Chile. Laboratorio de Dendrocronología y Cambio Global; Chile. Universidad de Chile. Center for Climate and Resilience Research; Chile Fil: Lézine, Anne-Marie. Université Pierre et Marie Curie. Laboratoire d’Océanographie et du Climat: Expérimentations et Approches Numériques (LOCEAN); Francia Fil: Ljungqvist, Fredrik C.. Stockholm University. Department of History, ; Suecia Fil: Lorrey, Andrew M.. National Institute of Water and Atmospheric Research Ltd., National Climate Centre Auckland; Nueva Zelanda Fil: Luterbacher, Jürg. Justus Liebig University. Department of Geography, Climatology, Climate Dynamics and Climate Change; Alemania Fil: Masson-Delmotte, Valérie . Laboratoire des Science du Climat et de l’Environnement, Gif-sur-Yvette; Francia Fil: McCarroll, Danny. Swansea University. Department of Geography; Reino Unido Fil: McConnell, Joseph R.. Nevada System of Higher Education. Desert Research Institute; Estados Unidos Fil: McKay, Nicholas P.. Northern Arizona University. School of Earth Sciences and Environmental Sustainability; Estados Unidos Fil: Morales, Mariano Santos. Consejo Nacional de Investigaciones Científicas y Técnicas. Científico Tecnológico Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Moy, Andrew D.. Australian Antarctic Division; Australia. University of Tasmania. Antarctic Climate & Ecosystems Cooperative Research Centre; Australia Fil: Mulvaney, Robert. British Antarctic Survey; Reino Unido Fil: Mundo, Ignacio Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Científico Tecnológico Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Nakatsuka, Takeshi. Nagoya University. Department of Earth and Environmental Sciences; Japón Fil: Nash, David J.. University of the Witwatersrand. School of Geography,Archaeology and Environmental Studies; Sudáfrica. University of Brighton. School of Environment and Technology; Reino Unido Fil: Neukom, Raphael . Swiss Federal Research Institute WSL; Suiza Fil: Nicholson, Sharon E.. Florida State University. Department of Earth, Ocean and Atmospheric Sciences, ; Estados Unidos Fil: Oerter, Hans. Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association. Department of Glaciology; Alemania Fil: Palmer, Jonathan G.. University of Exeter. College of Life and Environmental Sciences; Reino Unido. University of New South Wales. Climate Change Research Centre; Australia Fil: Phipps, Steven J.. University of New South Wales. Climate Change Research Centre; Australia. University of New South Wales. ARC Centre of Excellence for Climate System Science; Australia Fil: Prieto, Maria del Rosario. Consejo Nacional de Investigaciones Científicas y Técnicas. Científico Tecnológico Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Rivera, Andres. Centro de Estudios Cientificos; Chile Fil: Sano, Masaki. Nagoya University. Department of Earth and Environmental Sciences; Japón Fil: Severi, Mirko. University of Florence. Department of Chemistry ‘Ugo Schiff’; Italia Fil: Shanahan, Timothy M.. University of Texas at Austin. Jackson School of Geosciences; Estados Unidos Fil: Shao, Xuemei. Chinese Academy of Sciences. Institute of Geographical Sciences and Natural Resources Research; China Fil: Shi, Feng. Chinese Academy of Sciences. LASG, Institute of Atmospheric Physics, ; China Fil: Sigl, Michael . Nevada System of Higher Education. Desert Research Institute,; Estados Unidos Fil: Smerdon, Jason E.. Columbia University. Lamont Doherty Earth Observatory; Estados Unidos Fil: Solomina, Olga N. . Russian Academy of Sciences. Institute of Geography; Rusia Fil: Steig, Eric J.. University of Washington. Department of Earth and Space Sciences; Estados Unidos Fil: Stenni, Barbara. University of Trieste. Dipartimento di Matematica e Geoscienze; Italia Fil: Thamban, Meloth. National Centre for Antarctic and Ocean Research; India Fil: Trouet, Valerie. University of Arizona. Laboratory of Tree-Ring Research; Estados Unidos Fil: Turney, Chris S.M.. University of New South Wales. Climate Change Research Centre; Australia Fil: Umer, Mohammed. Addis Ababa University. School of Earth Sciences; Etiopía Fil: van Ommen, Tas. Australian Antarctic Division; Australia. University of Tasmania. Antarctic Climate & Ecosystems Cooperative Research Centre; Australia Fil: Verschuren, Dirk. Ghent University. Department of Biology; Bélgica Fil: Viau, Andre E.. University of Ottawa. Department of Geography; Canadá Fil: Villalba, Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Científico Tecnológico Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Vinther, Bo M.. University of Copenhagen. Niels Bohr Institute; Dinamarca Fil: von Gunten, Lucien . Past Global Changes (PAGES). International Project Office, ; Suiza Fil: Wagner, Sebastian. Helmholtz-Zentrum Geesthacht. Institute for Coastal Research; Alemania Fil: Wahl, Eugene R.. National Oceanic and Atmospheric Administration. National Climatic Data Center; Estados Unidos Fil: Wanner, Heinz. University of Bern. Oeschger Centre for Climate Change Research & Institute of Geography; Suiza Fil: Werner, Johannes P.. Justus Liebig University. Department of Geography, Climatology, Climate Dynamics and Climate Change; Alemania Fil: White, James W.C.. University of Colorado. Institute of Arctic and Alpine Research; Estados Unidos Fil: Yasue, Koh. Shinshu University. Department of Forest Science; Japón Fil: Zorita, Eduardo. Helmholtz-Zentrum Geesthacht. Institute for Coastal Research; Alemania

Water availability is fundamental to societies and ecosystems, but our understanding of variations 55 in hydroclimate (includingextreme events, flooding, and decadal periods of drought) is limited because of a paucity of modern instrumental observationsthat are distributed unevenly across the globe and only span parts of the 20th and 21st centuries. Such data coverage isinsufficient for characterizing hydroclimate and its associated dynamics because of its multidecadal-to-centennial variability andhighly regionalized spatial signature. High-resolution (seasonal to decadal) hydroclimatic proxies that span all or parts of the60 Common Era (CE) and paleoclimate model simulations are therefore important tools for augmenting our understanding ofhydroclimate variability. In particular, the comparison of the two sources of information is critical for addressing theuncertainties and limitations of both, while enriching each of their interpretations. We review the principal proxy data availablefor hydroclimatic reconstructions over the CE and highlight contemporary understanding of how these proxies are interpreted ashydroclimate indicators. We also review the available last-millennium simulations from fully-coupled climate models and65 discuss several outstanding challenges associated with simulating hydroclimate variability and change over the CE. A specificreview of simulated hydroclimatic changes forced by volcanic events is provided, as well as a discussion of expectedimprovements in estimated forcings, models and their implementation in the future. Our review of hydroclimatic proxies andlast-millennium model simulations is used as the basis for articulating a variety of considerations and best practices for how toperform proxy-model comparisons of CE hydroclimate. This discussion provides a framework for how best to evaluate70 hydroclimate variability and its associated dynamics using these comparisons, as well as how they can better informinterpretations of both proxy data and model simulations. We subsequently explore means of using proxy-model comparisonsto better constrain and characterize future hydroclimate risks. This is explored specifically in the context of several examplesthat demonstrate how proxy-model comparisons can be used to quantitatively constrain future hydroclimatic risks as estimatedfrom climate model projections. Fil: Smerdon, Jason E.. Columbia University; Estados Unidos Fil: Luterbacher, Jürg. University Of Giessen; Alemania Fil: Phipps, Steven J.. University of Tasmania; Australia Fil: Anchukaitis, Kevin J.. University of Arizona; Estados Unidos Fil: Ault, Toby. Cornell University; Estados Unidos Fil: Coats, Sloan. State University of Colorado at Boulder; Estados Unidos Fil: Cobb, Kim M.. Instituto Tecnológico de Georgia; Estados Unidos Fil: Cook, Benjamin I.. Nasa Goddard Institute For Space Studies; Estados Unidos Fil: Colose, Chris. Nasa Goddard Institute For Space Studies; Estados Unidos Fil: Felis, Thomas. Universitat Bremen; Alemania Fil: Gallant, Ailie. Monash University; Australia Fil: Jungclaus, Johann H.. Max-Planck-Institute for Meteorology; Alemania Fil: Konecky, Bronwen. State University of Colorado at Boulder; Estados Unidos Fil: LeGrande, Allegra. NASA Goddard Institute for Space Studie; Estados Unidos Fil: Lewis, Sophie. The Australian National University; Australia Fil: Lopatka, Alex S.. University of Maryland; Estados Unidos Fil: Man, Wenmin. Chinese Academy of Sciences; República de China Fil: Mankin, Justin S.. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos. NASA Goddard Institute for Space Studies; Estados Unidos Fil: Maxwell, Justin T.. Indiana University; Estados Unidos Fil: Otto-Bliesner, Bette L.. National Center for Atmospheric Research; Estados Unidos Fil: Partin, Judson W.. University of Texas at Austin; Estados Unidos Fil: Singh, Deepti. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Stevenson, Samantha. National Center for Atmospheric Research; Estados Unidos Fil: Tierney, Jessica E.. Arizona State University; Estados Unidos Fil: Zanchettin, Davide. Ca’ Foscari University of Venice; Italia Fil: Zhang, Huan. Justus Liebig University; Alemania Fil: Atwood, Alyssa R.. Georgia Institute of Technology; Estados Unidos. Brown University; Estados Unidos Fil: Andreu Hayles, Laia. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Baek, Seung H.. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Buckley, Brendan. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Cook, Edward. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: D'Arrigo, Rosanne. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Dee, Sylvia G.. Brown University; Estados Unidos Fil: Griffiths, Michael L.. William Paterson University; Estados Unidos Fil: Kulkarni, Charuta. City University of New York; Estados Unidos Fil: Kushnir, Yochanan. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Lehner, Flavio. National Center for Atmospheric Research; Estados Unidos Fil: Leland, Caroline. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Linderholm, Hans W.. University of Gothenburg; Suecia Fil: Okazaki, Atsushi. University of Tokyo; Japón Fil: Palmer, Jonathan. University of New South Wales; Australia Fil: Piovano, Eduardo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina Fil: Raible, Christoph C.. University of Bern; Suiza Fil: Rao, Mukund P.. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Scheff, Jacob. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Schmidt, Gavin A.. NASA Goddard Institute for Space Studie; Estados Unidos Fil: Seager, Richard. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Widmann, Martin. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Park Williams, A.. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Xoplaki, Elena. Justus Liebig University; Alemania

Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative. The database gathers 692 records from 648 locations, including all continental regions and major ocean basins. The records are from trees, ice, sediment, corals, speleothems, documentary evidence, and other archives. They range in length from 50 to 2000 years, with a median of 547 years, while temporal resolution ranges from biweekly to centennial. Nearly half of the proxy time series are significantly correlated with HadCRUT4.2 surface temperature over the period 1850-2014. Global temperature composites show a remarkable degree of coherence between high- and low-resolution archives, with broadly similar patterns across archive types, terrestrial versus marine locations, and screening criteria. The database is suited to investigations of global and regional temperature variability over the Common Era, and is shared in the Linked Paleo Data (LiPD) format, including serializations in Matlab, R and Python., PAGES, a core project of Future Earth, is supported by the U.S. and Swiss National Science Foundations. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Some of this work was conducted as part of the North America 2k Working Group supported by the John Wesley Powell Center for Analysis and Synthesis, funded by the U.S. Geological Survey. B. Bauer, W. Gross, and E. Gille (NOAA National Centers for Environmental Information) are gratefully acknowledged for helping assemble the data citations and creating the NCEI versions of the PAGES 2k data records. We thank all the investigators whose commitment to data sharing enables the open science ethos embodied by this project.

Trees, 27 (4), ISSN:0931-1890, ISSN:1432-2285

Center for Accelerator Mass Spectrometry (CAMS) Tasmanian Huon pine (Lagarostrobos franklinii) decadal measurements for the interval AD 745–855 suggest a mean interhemispheric radiocarbon offset (20 ± 5 yr), which is considerably lower than the previously reported mean interhemispheric offset for the last 2 millennia (44 ± 17 yr). However, comparable University of Waikato (Wk) New Zealand kauri (Agathis australis) measurements show significantly higher values (56 ± 6 yr), suggesting the possibility of a temporary geographic (intrahemispheric) offset between Tasmania, Australia, and Northland, New Zealand, during at least 1 common time interval. Here, we report 9 new Wk Tasmanian Huon pine measurements from the decades showing the largest Huon/kauri difference. We show statistically indistinguishable Wk Huon and Wk kauri 14C ages, thus dispelling the suggestion of a 14C geographic offset between Tasmania and Northland.

Eastern Australia recently experienced an intense drought (Millennium Drought, 2003-2009) and record-breaking rainfall and flooding (austral summer 2010-2011). There is some limited evidence for a climate change contribution to these events, but such analyses are hampered by the paucity of information on long-term natural variability. Analyzing a new reconstruction of summer (December-January-February) Palmer Drought Severity Index (the Australia-New Zealand Drought Atlas; ANZDA, 1500-2012 CE), we find moisture deficits during the Millennium Drought fall within the range of the last 500 years of natural hydroclimate variability. This variability includes periods of multi-decadal drought in the 1500s more persistent than any event in the historical record. However, the severity of the Millennium Drought, which was caused by autumn (March-April-May) precipitation declines, may be underestimated in the ANZDA because the reconstruction is biased towards summer and antecedent spring (September-October-November) precipitation. The pluvial in 2011, however, which was characterized by extreme summer rainfall faithfully captured by the ANZDA, is likely the wettest year in the reconstruction for Coastal Queensland. Climate projections (RCP 8.5 scenario) suggest that eastern Australia will experience long-term drying during the 21st century. While the contribution of anthropogenic forcing to recent extremes remains an open question, these projections indicate an amplified risk of multi-year drought anomalies matching or exceeding the intensity of the Millennium Drought.

Multidecadal droughts that occurred during the Medieval Climate Anomaly represent an important target for validating the ability of climate models to adequately characterize drought risk over the near-term future. A prominent hypothesis is that these megadroughts were driven by a centuries-long radiatively forced shift in the mean state of the tropical Pacific Ocean. Here we use a novel combination of spatiotemporal tree-ring reconstructions of Northern Hemisphere hydroclimate to infer the atmosphere-ocean dynamics that coincide with megadroughts over the American West, and find that these features are consistently associated with ten-to-thirty year periods of frequent cold El Nino Southern Oscillation conditions and not a centuries-long shift in the mean of the tropical Pacific Ocean. These results suggest an important role for internal variability in driving past megadroughts. State-of-the art climate models from the Coupled Model Intercomparison Project phase 5, however, do not simulate a consistent association between megadroughts and internal variability of the tropical Pacific Ocean, with implications for our confidence in megadrought risk projections.

Provenance studies are an increasingly important analog for understanding how trees adapted to particular climatic conditions might respond to climate change. Dendrochronological analysis can illuminate differences among trees from different seed sources in terms of absolute annual growth and sensitivity to external growth factors. We analyzed annual radial growth of 567 36-year-old pitch pine (Pinus rigida Mill.) trees from 27 seed sources to evaluate their performance in a New Jersey Pine Barrens provenance experiment. Unexpectedly, missing rings were prevalent in most trees, and some years—1992, 1999, and 2006—had a particularly high frequency of missing rings across the plantation. Trees from local seed sources (, PLoS ONE, 11 (5), ISSN:1932-6203

We develop a summer temperature reconstruction for temperate East Asia based on a network of annual tree-ring chronologies covering the period 800–1989 C.E. The East Asia reconstruction is the regional average of 585 individual grid point summer temperature reconstructions produced using an ensemble version of point-by-point regression. Statistical calibration and validation tests indicate that the regional average possesses sufficient overall skill to allow it to be used to study the causes of temperature variability and change over the region. The reconstruction suggests a moderately warm early medieval epoch (ca. 850–1050 C.E.), followed by generally cooler ‘Little Ice Age’ conditions (ca. 1350–1880 C.E.) and 20th century warming up to the present time. Since 1990, average temperature has exceeded past warm epochs of comparable duration, but it is not statistically unprecedented. Superposed epoch analysis reveals a volcanic forcing signal in the East Asia summer temperature reconstruction, resulting in pulses of cooler summer conditions that may persist for several years. Substantial uncertainties remain, however, particularly at lower frequencies, thus requiring caution and scientific prudence in the interpretation of this record.

Growing scientific evidence from modern climate science is loaded with implications for the environmental history of the Roman Empire and its successor societies. The written and archaeological evidence, although richer than commonly realized, is unevenly distributed over time and space. A first synthesis of what the written records and multiple natural archives (multi-proxy data) indicate about climate change and variability across western Eurasia from c. 100 b.c. to 800 a.d. confirms that the Roman Empire rose during a period of stable and favorable climatic conditions, which deteriorated during the Empire's third-century crisis. A second, briefer period of favorable conditions coincided with the Empire's recovery in the fourth century; regional differences in climate conditions parallel the diverging fates of the eastern and western Empires in subsequent centuries. Climate conditions beyond the Empire's boundaries also played an important role by affecting food production in the Nile valley, and by encouraging two major migrations and invasions of pastoral peoples from Central Asia.

Drought patterns across monsoon and temperate Asia over the period 1877���2005 are linked to Indo-Pacific climate variability associated with the El Ni��o-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). Using the Monsoon Asia Drought Atlas (MADA) composed of a high-resolution network of hydroclimatically sensitive tree-ring records with a focus on the June���August months, spatial drought patterns during El Ni��o and IOD events are assessed as to their agreement with an instrumental drought index and consistency in the drought response amongst ENSO/IOD events. Spatial characteristics in drought patterns are related to regional climate anomalies over the Indo-Pacific basin, using reanalysis products, including changes in the Asian monsoon systems, zonal Walker circulation, moisture fluxes, and precipitation. A weakening of the monsoon circulation over the Indian subcontinent and Southeast Asia during El Ni��o events, along with anomalous subsidence over monsoon Asia and reduced moisture flux, is reflected in anomalous drought conditions over India, Southeast Asia and Indonesia. When an IOD event co-occurs with an El Ni��o, severe drought conditions identified in the MADA for Southeast Asia, Indonesia, eastern China and central Asia are associated with a weakened South Asian monsoon, reduced moisture flux over China, and anomalous divergent flow and subsidence over Indonesia. Insights into the relative influences of Pacific and Indian Ocean variability for Asian monsoon climate on interannual to decadal and longer timescales, as recorded in the MADA, provide a useful tool for assessing long-term changes in the characteristics of Asian monsoon droughts in the context of Indo-Pacific climate variability.

Uncertainty related to the rate and magnitude of predicted anthropogenic climate change highlights the need to enhance our understanding of past natural fluctuations in the Earth's climate system. This task emphasizes the importance of high-resolution palaeoclimatic records that cover industrial and pre-industrial times. Annually resolved and absolutely dated tree-ring measurements are a key input for cross-disciplinary research. Ambiguity due to paucity of data, however, characterizes many tree-ring data analyses. By utilizing nearly 12 000 living and historical ring width series from European fir Abies alba Mill., we demonstrate how massive sam- ple replication can generate robust estimates of past growth rates, which may help reduce methodological and statistical constraints associated with many traditional tree-ring studies.

Understanding spatiotemporal tree growth variability and its associations with climate can provide key insights into forest dynamics in the context of global climate change. Here, we conduct a comprehensive investigation on 64 ring-width chronologies across the entire Northwest (NW) China to understand the regional patterns of tree growth and climate–growth relationships. Using rotated principal component analysis and hierarchical clustering analysis, we found that tree growth was mainly determined by the climate and could be classified into nine groups. Most of the tree-ring chronologies in NW China showed high correlations with moisture conditions in the current and previous growing seasons. After removing age-related growth trends, inter-annual tree growth patterns are supposed to be mainly determined by climate and climate–growth relationships. Since climate–growth relationships for most tree-ring chronologies in this arid region are similar, patterns of tree growth are mainly determined by climate variability. Within each group, the strength of the common signal increases under extreme climate conditions. Thus, climate plays a more important role in determining tree growth in extreme climate conditions relative to the non-climate factors, leading to more coherent growth patterns.

article i nfo We describe a millennial length (~1500-yr) tree-ring chronology developed from West Virginia (WVA), USA red cedar (Juniperus virginiana) ring widths that is significantly correlated with local to regional temperature and precipitation for the region. Using ensemble methods of tree-ring standardization, above average ring widths are indicated for the period between ~1000 and 1300 CE, the approximate time of the Medieval Climate Anomaly (MCA), the most recent major warm episode prior to the modern era. The chronology then transitions to more negative overall growth persisting through much of the subsequent period known as the Little Ice Age (LIA). While WVA cedar growth levels during the MCA are broadly similar to the 20th century mean, the most positive values during the MCA are associated with RCS-standardized chronologies, which pseudoproxy tests reveal are likely biased artificially positive, warranting further investigation. This cedar record is significantly correlated with the NAO, due to the tendency for warmer, wetter conditions to occur in the eastern-central USA during the NAO's positive phase. These types of conditions are inferred for this cedar chronology during the MCA period, during which NAO reconstructions suggest a persistently-positive NAO state.

This paper presents a multicentury reconstruction of May precipitation (1200–1997) for the mid-Atlantic region of the United States. The reconstruction is based on the first principal component (PC1) of two millennial-length Juniperus virginiana L. (eastern red cedar) tree-ring chronologies collected from rocky, limestone sites in the Ridge and Valley province of West Virginia. A split-calibration linear regression model accounted for 27% of the adjusted variance in the instrumental record and was stable through time. The model was verified by the reduction of error (RE = 0.21) and coefficient of efficiency (CE = 0.20) statistics. Multidecadal changes in precipitation were common throughout the reconstruction, and wetter than median conditions and drier than median conditions occurred during the medieval climate anomaly (1200–1300) and the Little Ice Age (1550–1650), respectively. The full reconstruction contained evidence of interannual and decadal variability; however, the twentieth century recorded the greatest number of decadal extreme wet and dry periods. A comparison of the May precipitation reconstruction to other regional reconstructions [Potomac River, Maryland, streamflow (Cook and Jacoby); Virginia/North Carolina July Palmer hydrologic drought index (PHDI; Stahle et al.); Missouri July PHDI (Cleaveland and Stahle); and White River, Arkansas, streamflow (Cleaveland)] showed that the eastern U.S. decadal drought and pluvial events extended into the mid-Atlantic region. A positive correlation between PC1 and the winter North Atlantic Oscillation (NAO) index and comparisons of smoothed May precipitation and the NAO (Luterbacher et al.) indicated that J. virginiana’s response to May precipitation was mediated by winter temperature.

A preliminary study of a point-by-point spatial precipitation reconstruction for northwestern (NW) China is explored, based on a tree-ring network of 132 chronologies. Precipitation variations during the past ~200–400 yr (the common reconstruction period is from 1802 to 1990) are reconstructed for 26 stations in NW China from a nationwide 160-station dataset. The authors introduce a “search spatial correlation contour” method to locate candidate tree-ring predictors for the reconstruction data of a given climate station. Calibration and verification results indicate that most precipitation reconstruction models are acceptable, except for a few reconstructions (stations Hetian, Hami, Jiuquan, and Wuwei) with degraded quality. Additionally, the authors compare four spatial precipitation factors in the instrumental records and reconstructions derived from a rotated principal component analysis (RPCA). The northern and southern Xinjiang factors from the instrumental and reconstructed data agree well with each other. However, differences in spatial patterns between the instrumentation and reconstruction data are also found for the other two factors, which probably result from the relatively poor quality of a few stations. Major drought events documented in previous studies—for example, from the 1920s through the 1930s for the eastern part of NW China—are reconstructed in this study.

Our ability to predict El Nino/Southern Oscillation (ENSO) activity is hampered by the relatively short length of the instrumental record. An annually resolved record of ENSO variability over the past millennium based on tree rings indicates that ENSO amplitude varies on a 50–90 year cycle, providing an important constraint for improving predictions. The El Nino/Southern Oscillation (ENSO) is the dominant mode of interannual climate variability on Earth, alternating between anomalously warm (El Nino) and cold (La Nina) conditions in the tropical Pacific at intervals of 2–8 years1,2. The amplitude of ENSO variability affects the occurrence and predictability of climate extremes around the world3,4, but our ability to detect and predict changes in ENSO amplitude is limited by the fact that the instrumental record is too short to characterize its natural variability5,6,7,8. Here we use the North American Drought Atlas9,10—a database of drought reconstructions based on tree-ring records—to produce a continuous, annually resolved record of ENSO variability over the past 1,100 years. Our record is in broad agreement with independent, ENSO-sensitive proxy records in the Pacific and surrounding regions. Together, these records indicate that ENSO amplitude exhibits a quasi-regular cycle of 50–90 years that is closely coupled to the tropical Pacific mean state. Anomalously warm conditions in the eastern Pacific are associated with enhanced ENSO variability, consistent with model simulations11. The quasi-periodic ENSO amplitude modulation reported here offers a key observational constraint for improving models and their prediction of ENSO behaviour linked to global warming.

Climate information may help smallholder farmers in Southeast Asia takeinformed risks and reduce their climate vulnerability. Model-based seasonalforecasts are limited in the skill they offer with longer lead times, and intheir accessibility to farmers in remote regions. Climate reconstructions derivedfrom tree rings can complement model-based forecasts, providing probabilisticestimates of future climate at an annual resolution. This study demonstratesthe skill of PDSI reconstructions derived from the Monsoon Asia DroughtAtlas (MADA) to forecast a climate signal several years into the future, andillustrates the advantage of such information vis-a-vis the simple strategy ofassuming climate to persist from year to year. This work shows the potentialfor such climate reconstructions to improve inter-annual decisions on SoutheastAsian farms, and highlights the social research necessary to transmute thispotential into a valuable climate decision aid.In much of Southeast Asia the capacity of smallholder farmers to predict andprotect against drought is limited. Farmers commonly employ conservative, risk-averse strategies to farm planning and operation that reduce losses to climate-relateddamages, but that also reduce surpluses in years of favorable climate (Meinke andStone 2005). Farmers operating close to thresholds of sustenance and self-sufficiencywho fail to take advantage of these rents can fall into ‘poverty traps’ (Holling 2001;Sachs 2005), unable to lift themselves from degraded land conditions and a lackof capital. To help smallholders become informed risk takers and capitalize on the

article i nfo For drought and flood forecasting, it is critical that we understand regional rainfall patterns as well as their potential underlying forcings, particularly in urban regions with greater long-term anthropogenic influences compared to rural areas. To achieve this, three Chinese pine (Pinus tabulaeformis) tree-ring width chronologies were developed from the metropolitan, suburban, and rural sites near Shenyang city of Northeast China and used to study regional dryness and wetness variability in the past 232 yr. January-May precipitation (1771-2002) was successfully reconstructed from the tree-rings, although urban factors caused a slightly reduced response of the tree-rings to precipitation. Our reconstruction captures certain local rainfall variability and dryness/wetness signals, such as three unusually dry decades (1920s, 1850s and 1960s), an unusually dry half-century (1850-1899) and some significant cycles (2-8 yr and ~20 yr) associated with them. It also reveals a relatively equal number of dry/wet events (38.79% wet years versus 35.78% dry years) and a recent drying trend. Our study also indicates the feasibility of using urban tree-rings to reconstruct dry/ wet patterns in city environments.

Occurrence of drought and dry periods in southeastern Australia has been linked to broad scale climate phenomena including the Southern Oscillation, Interdecadal Pacific Oscillation (IPO), Indian Ocean Dipole (IOD), Southern Annular Mode (SAM) and persistence of blocking high pressure in the Tasman Sea. We examine relationships between Athrotaxis tree ring chronologies from southern Australia extending over much of the past millennia and these broad scale indices. We also examine relationships between the chronologies, temperature, precipitation and a standardised precipitation and evapotranspiration index. Timing of significant correlations with maximum temperature varies between species. The responses of the species with broadscale indices vary with location: northern Athrotaxis cupressoides (Pencil Pine) are more strongly related to the Interdecadal Pacific Oscillation (IPO) and Southern Oscillation Index (SOI) than southern sites. As an exception to this, a site in the far south had significant correlations with both the SOI and IPO, opposite in sign to those observed for the northern sites. Significant spectral power at frequencies consistent with the SOI and IPO occur in all chronologies. Western and southern sites are more strongly related to a seasonalised index of SAM. These three systems have played important roles in determining moisture conditions in southeastern Australia over the past millennium. Results suggest that reconstructions of the SOI, IPO or SAM are unlikely based solely on this Athrotaxis network. The Athrotaxis network of tree ring sites, is however, likely to be an important input to multi-proxy models reconstructing the SOI, IPO or SAM in the Australian sector. The Athrotaxis network of sites is also an important extension of the existing network of Australian tree ring sites that could be used to reconstruct historical drought in southeastern Australia.

Drought is among the most costly natural hazards affecting the United States, averaging $6 to $8 billion annually in damages, primarily in crop losses. Mitigating the impacts of drought through planning and preparedness has the potential to save billions of dollars. We used a new long tree-ring chronology developed from the central U.S. to reconstruct annual drought and characterize past drought duration, frequency, and cycles in the U.S. Corn Belt region during the last millennium. This is the first paleoclimate reconstruction achieved with subfossil oak wood in the U.S. and increases the current dendroclimatic record in the central U.S. agricultural region by over 500 years. A tree ring-width drought response function was calibrated and verified against monthly instrumental Palmer Hydrologic Drought Index (PHDI) during the summer season (JJA). Separate reconstructions tailored to emphasize high-frequency and low-frequency variations indicate that drought conditions over the period of instrumental records (since 1895) do not exhibit the full range of variability, severity, or duration of droughts during the last millennium. For example, three years in the last millennium were drier than 1934, a classic Dust-Bowl year and the driest year of the instrumental period. Thirteen decadal to multidecadal droughts (i.e., ≥10 years) occurred during the last millennium – the longest lasting sixty-one years and centered on the late twelfth century. Reconstructions exhibited quasi-periodicity at bidecadal and century-scale periods. Significant rhythms in drought were identified near 20-yr and 128-yr periods. The tree-ring drought reconstruction shows promise in providing new information about long-term climate variability in the agricultural regions that could potentially span multimillennia. We postulate that tree-ring chronologies (i.e., tree growth), thus far under-utilized in agricultural applications, have the potential to match contributions of instrumental climate data.

A collection of 28 tree-ring chronologies from six different species located in northern Pakistan were evaluated in terms of their potential for dendroclimatic reconstructions. 15 of the sites are new while the remaining 13 (all Juniperus excelsa M. Bieb.) have been reported earlier. Several species had trees attaining ages of around 700 years (Cedrus deodara (D. Don) G. Don, Pinus gerardiana Wall. ex D. Don., Pinus wallichiana A.B. Jacks and Picea smithiana (Wall.) Boiss.) but the juniper was clearly the oldest with some trees greater than 1000 years. Correlations between the site chronologies declined with increasing separation distance. This was consistently seen both between sites of the same species and between sites composed of different species. This led to a situation where a much stronger correlation occurred between two different species growing at the same site than between sites of the same species but separated by as little as 0.5 km. Such results highlight the obvious strong elevational gradients present in this mountainous region (where some elevations are over 7000 m). They also lend support to the practice of multi-species combinations for better spatial and temporal coverage. The best prospects for this appear to be C. deodara and P. gerardiana and are consistent with studies from neighbouring India. The comparison to 0.5 ◦ gridded climate data was strongest from the same two species though P. smithiana at one site was also highly significant. A general climate correlation pattern from all species was evident that starts with a strong negative relationship to temperature in the previous October, then turns towards positive during winter, before again becoming significantly negative by the current May. The previous October signal is thought to be a lag effect where hot temperatures (and low soil-moisture) stress the trees, thereby reducing reserves available for the following spring. Similarly, hot temperatures in late spring (May) lead to greater soil moisture losses and tree transpiration costs. Conversely, there is an extended strong positive precipitation correlation from late winter to spring (January–May). This ends abruptly and there is no evidence of a summer (June–September) monsoon signal seen in the rainfall correlation functions.

A key feature of anticipated 21st century droughts in Southwest North America is the concurrence of elevated temperatures and increased aridity. Instrumental records and paleoclimatic evidence for past prolonged drought in the Southwest that coincide with elevated temperatures can be assessed to provide insights on temperature-drought relations and to develop worst-case scenarios for the future. In particular, during the medieval period, ∼AD 900–1300, the Northern Hemisphere experienced temperatures warmer than all but the most recent decades. Paleoclimatic and model data indicate increased temperatures in western North America of approximately 1 °C over the long-term mean. This was a period of extensive and persistent aridity over western North America. Paleoclimatic evidence suggests drought in the mid-12th century far exceeded the severity, duration, and extent of subsequent droughts. The driest decade of this drought was anomalously warm, though not as warm as the late 20th and early 21st centuries. The convergence of prolonged warming and arid conditions suggests the mid-12th century may serve as a conservative analogue for severe droughts that might occur in the future. The severity, extent, and persistence of the 12th century drought that occurred under natural climate variability, have important implications for water resource management. The causes of past and future drought will not be identical but warm droughts, inferred from paleoclimatic records, demonstrate the plausibility of extensive, severe droughts, provide a long-term perspective on the ongoing drought conditions in the Southwest, and suggest the need for regional sustainability planning for the future.

Moisture sensitive trees from the central Tien Shan area, northwest China show a pattern of extremely low growth in 1945. Evidence from a number of sources indicates that a remarkable drought event, in both spatial and temporal terms, occurred over the central Tien Shan and surrounding areas in that year. Climatological analysis reveals that this significant growth reduction is likely a combined effect of unusually dry and warm conditions. We also show that this extreme drought is associated with a La Nina or cold phase El Nino-Southern Oscillation (ENSO) event. Meanwhile, cooler sea surface temperatures (SSTs) in the northern Indian Ocean, particularly in the Arabian Sea, also appear to have contributed to this severe drought episode. Overall, our case study of 1945 extreme drought event in the central Tien Shan area suggests there might be a close link between extreme climate conditions in remote north central Asia and coupled ocean-atmosphere dynamics in the tropics.

We examine how the seasonality of precipitation signals embedded within the North American Drought Atlas varies across the continent. Instrumental records of average summer (JJA) Palmer Drought Severity Index (PDSI) are characterized by major regional differences in the relative importance of precipitation during summer and winter (DJF). The Atlas, which is based on a network of drought-sensitive tree-ring records, is able to reproduce the main geographic patterns of these biases, but tree-ring reconstructions exaggerate the influence of seasonal precipitation anomalies in the southwestern United States and northern Mexico (towards a stronger winter signal) and western Canada (towards a stronger summer signal). Drought reconstructions from the Southwest and Tex-Mex regions are tuned mainly to winter precipitation and display strong teleconnections to both El Niño and La Niña. In contrast, winter precipitation signals are either weak or absent in drought reconstructions from northwestern North America, and tree-ring estimates of PDSI show a much less robust association with the El Niño-Southern Oscillation. Geographical differences in the relative strength of seasonal precipitation signals are likely due to (i) local factors that influence tree growth but are not incorporated into the PDSI algorithm and (ii) real differences in regional climatology. These seasonal biases must be taken into account when comparing drought reconstructions across North America, when comparing tree-ring PDSI to drought records developed from other proxies or when attempting to use the Drought Atlas to link past droughts to potential forcing mechanisms.

Of Monsoons and Megadroughts The Asian monsoon is the weather system that has the greatest effect on the greatest number of people in the world. Naturally then, knowing better how climate change might affect the monsoon is tremendously important. One obstacle that prevents a better understanding of future behavior is a poor knowledge of its past. Cook et al. (p. 486 ; see the Perspective by Wahl and Morrill ) help to fill this gap with a 700-year reconstruction of the monsoon from tree-ring data obtained throughout Asia. The reconstruction chronicles monsoon failures and megadroughts, as well as patterns of precipitation, and can thus be compared with other relevant climate records to allow links with sea-surface temperatures to be better understood.

Two robust tree-ring width chronologies were developed for the western Tianmu Moun- tains and the Xianyu Mountains of southeast China. Both chronologies were significantly correlated with each other and were arithmetically averaged to build a regional chronology (RC). The RC had significant and positive correlations with winter temperature before the growing season. Based on this relationship, the average temperatures of the previous December to the current March were reconstructed using the RC chronology for the period 1852 to 2006. The temperature reconstruction was significantly correlated with the winter half-year temperature in the eastern Qinling Mountains of central China, 720 km west of the study region, suggesting a large-scale coherence of winter tem- perature variability. The reconstruction corresponds well with an East Asian winter monsoon (EAWM) index in extreme years and reflects the strong influences of the EAWM in this study region. Thus, there is great potential to use tree-ring chronologies to reconstruct past climate change in southeast China, where little dendroclimatic work has been done until now.

The “hydraulic city” of Angkor, the capitol of the Khmer Empire in Cambodia, experienced decades-long drought interspersed with intense monsoons in the fourteenth and fifteenth centuries that, in combination with other factors, contributed to its eventual demise. The climatic evidence comes from a seven-and-a-half century robust hydroclimate reconstruction from tropical southern Vietnamese tree rings. The Angkor droughts were of a duration and severity that would have impacted the sprawling city’s water supply and agricultural productivity, while high-magnitude monsoon years damaged its water control infrastructure. Hydroclimate variability for this region is strongly and inversely correlated with tropical Pacific sea surface temperature, indicating that a warm Pacific and El Niño events induce drought at interannual and interdecadal time scales, and that low-frequency variations of tropical Pacific climate can exert significant influence over Southeast Asian climate and society.

Reliable, long-term records of daily weather and climate are relatively rare but are crucial for understanding long-term trends and variability in extreme events and other climate metrics that are not resolvable at the monthly time scale. Here, the distinct features of a continuous, long-term (1896–2006) daily weather record from Mohonk Lake, New York, are highlighted. The site is optimal for daily climate analyses, since it has experienced negligible land-use change, no station moves, and has maintained methodological and instrumental consistency over the entire period of record. Unlike many sites, the site has always used maximum/minimum thermometers rather than shifting to the automated Maximum/Minimum Temperature Sensor. Notable results from the analysis of this record include 1) a warming trend driven largely by trends in maximum temperatures, especially during summer, 2) increasing diurnal temperature range during summer, and 3) a reduction in the number of freeze-days per year with little change in the length of the freeze-free season. These findings deviate from some regional level trends, suggesting there may be value in revisiting selected, consistently monitored, and maintained stations similar to Mohonk for focused analyses of regional climate change.

Spatial reconstructions of drought for central High Asia based on a tree-ring network are presented. Drought patterns for central High Asia are classified into western and eastern modes of variability. Tree-ring based reconstructions of the Palmer drought severity index (PDSI) are presented for both the western central High Asia drought mode (1587–2005), and for the eastern central High Asia mode (1660–2005). Both reconstructions, generated using a principal component regression method, show an increased variability in recent decades. The wettest epoch for both reconstructions occurred from the 1940s to the 1950s. The most extreme reconstructed drought for western central High Asia was from the 1640s to the 1650s, coinciding with the collapse of the Chinese Ming Dynasty. The eastern central High Asia reconstruction has shown a distinct tendency towards drier conditions since the 1980s. Our spatial reconstructions agree well with previous reconstructions that fall within each mode, while there is no significant correlation between the two spatial reconstructions.

Decadal samples of dendrochronologically dated pine (Lagorostrobos franklinii) from the Stanley River basin, Tasmania, have been radiocarbon dated between 2120–850 yr BP. This data set overlaps and extends the current Southern Hemisphere record, which covers the period 110–995 yr BP. There is good agreement between the 2 records between 995–850 yr BP, between sample replicates and with consensus values for standards. As in the younger data set, we find evidence for a distinct but variable offset between the Southern Hemisphere data and IntCal04; although this is likely due to real temporal variability in the interhemispheric offset, further work is planned to rule out possible laboratory or sample preparation differences.

In this study we compare three newly developed independent NINO3.4 sea surface temperature (SST) reconstructions using data from (1) the central Pacific (corals), (2) the TexMex region of the USA (tree rings) and (3) other regions in the Tropics (corals and an ice core) which are teleconnected with central Pacific SSTs in the 20th century. Although these three reconstructions are strongly calibrated and well verified, inter-proxy comparison shows a significant weakening in inter- proxycoherenceinthe 19thcentury.Thisbreakdownin commonsignalcouldberelatedtoinsufficient data, dating errors in some of the proxy records or a breakdown in El Nino-Southern Oscillation's (ENSO's) influence on other regions. However, spectral analysis indicates that each reconstruction portraysENSO-likespectralproperties.Superposedepochanalysisalsoshowsthateachreconstruction shows a generally consistent 'El Nino-like' response to major volcanic events in the following year, while duringyearsT þ4toT þ7'La Nina-like'conditions prevail.Theseresults suggest thateachofthe series expresses ENSO-like 'behaviour', but this 'behaviour' does not appear to be spatially or temporally consistent. This result may reflect published observations that there appear to be distinct 'types' of ENSO variability depending on location within the tropical Pacific. Future work must address potential dating issues within some proxies (i.e. sampling of multiple coral heads for one location) as well as assessing the time stability of local climate relationships with central Pacific SSTs. More emphasis is needed on sampling new and extending old coral proxy records from the crucial central and eastern tropical Pacific region. Copyright # 2009 John Wiley & Sons, Ltd. and # Crown Copyright 2009.

Droughts, which occur as a part of natural climate variability, are expected to increase in frequency and/or severity with global climate change. An improved understanding of droughts and their association with atmospheric circulation will add to the knowledge about the controls on drought, and the ways in which changes in climate may impact droughts. In this study, 1) major drought patterns across the United States have been defined, 2) the robustness of these patterns over time using tree-ring-based drought reconstructions have been evaluated, and 3) the drought patterns with respect to global atmospheric pressure patterns have been assessed. From this simple assessment, it is suggested that there are two major drought patterns across North America, which together account for about 30% of the total variance in drought patterns—one resembles the classic ENSO teleconnection, and the other displays an east–west drought dipole. The same two patterns are evident in the instrumental data and the reconstructed drought data for two different periods, 1404–2003 and 900–1350. The 500-mb circulation patterns associated with the two drought patterns suggest that the controls on drought may come from both Northern Hemisphere and tropical sources. The two drought patterns, and presumably their associated circulation patterns, vary in strength over time, indicating the combined effects of the two patterns on droughts over the past millennium.

IPCC Assessment Report 4 model projections suggest that the subtropical dry zones of the world will both dry and expand poleward in the future due to greenhouse warming. The US Southwest is particularly vulnerable in this regard and model projections indicate a progressive drying there out to the end of the 21st century. At the same time, the USA has been in a state of drought over much of the West for about 10 years now. While severe, this turn of the century drought has not yet clearly exceeded the severity of two exceptional droughts in the 20th century. So while the coincidence between the turn of the century drought and projected drying in the Southwest is cause for concern, it is premature to claim that the model projections are correct. At the same time, great new insights into past drought variability over North America have been made through the development of the North American Drought Atlas from tree rings. Analyses of this drought atlas have revealed past megadroughts of unprecedented duration in the West, largely in the Medieval period about 1000 years ago. A vastly improved Living Blended Drought Atlas (LBDA) for North America now under develop- ment reveals these megadroughts in far greater detail. The LBDA indicates the occurrence of the same Medieval megadroughts in the West and similar-scale megadroughts in the agriculturally and commercially important Mississippi Valley. Possible causes of these megadroughts and their implica- tions for the future are discussed. Copyright # 2009 John Wiley & Sons, Ltd.

We present a Palmer Drought Severity Index reconstruction (r = 0.61, P < 0.01) from 1440 to 2007 for the southeastern Tibetan Plateau, based on tree rings of the forest fir (Abies forrestii). Persistent decadal dry intervals were found in the 1440s-1460s, 1560s-1580s, 1700s, 1770s, 1810s, 1860s and 1980s, and the extreme wet epochs were the 1480s-1490s, 1510s-1520s, 1590s, 1610s-1630s, 1720s-1730s, 1800s, 1830s, 1870s, 1930s, 1950s and after the 1990s. Comparisons of our record with those identified in other moisture related reconstructions for nearby regions showed that our reconstructed droughts were relatively consistent with those found in other regions of Indochina, suggesting similar drought regimes. Spectral peaks of 2.3-5.5 years may be indicative of ENSO activity, as also suggested by negative correlations with SSTs in the eastern equatorial and southeastern Pacific Ocean. Significant multidecadal spectral peaks of 29.2-40.9 and 56.8-60.2 years were identified. As indicated by the spatial correlation patterns, the decadal-scale variability may be linked to SST variations in the northern Pacific and Atlantic Oceans.

During the early Mississippian Lohmann phase (A.D. 1050-1100), the American Bottom experienced a political and economic transformation. This transformation included the abrupt planned construction of central Cahokia, a large-scale influx of people to "downtown Cahokia," the abandonment of pre-Mississippian village settlements, the reorganization of farming in the Mississippi River floodplain, and the founding of the Richland farming complex in the Illinois uplands. New tree-ring-based records of climate change indicate that this rapid development occurred during one of the wettest 50-year periods during the last millennium. During the next 150 years, a series of persistent droughts occurred in the Cahokian area which may be related to the eventual abandonment of the American Bottom. By A.D. 1150, in the latter part of a severe 15-year drought, the Richland farming complex was mostly abandoned, eliminating an integral part of Cahokia$s agricultural base. At about the same time, a 20,000-log palisade was erected around Monks Mound and the Grand Plaza, indicating increased social unrest. During this time, people began exiting Cahokia and, by the end of the Stirling phase (A.D. 1200), Cahokia$s population had decreased by about 50 percent and by A.D. 1350, Cahokia and much of the central Mississippi valley had been abandoned.

Western China, defined here as the land falling within the geographical boundaries of the People's Republic of China in the west, south and north, and the approximate eastern extent of the Tibetan and Alaxa Plateaux to the east, occupies an important climatic region, influenced by the Asian and Indian summer monsoons, the mid-latitude westerlies and the dry, cold central Asian winter monsoon. The Tibetan Plateau itself is a prominent topographic feature that exerts major control on regional atmospheric circulation. Previous compilations of meteorological data and documentary sources suggest that western China, and the Tibetan Plateau in particular, is highly sensitive to anthropogenically induced climate change. Temperature increases appear to be greatest at higher altitudes: moreover, precipitation variations seem to have been marked, although spatially complex. The region contains a wealth of information about past climate derived from instrumental, documentary and proxy sources although meteorological time series are generally too short to capture the full range of recent climatic variability. Documentary and proxy sources are therefore important. We review studies of climate change in western China for the past two millennia. Documentary records are complemented by proxy data from ice cores, tree rings, lake sediments, groundwater profiles and glacial geomorphology. Although general patterns of change can be identified, proxy records of past climate are often semi-quantitative at best, open to alternative interpretations and sometimes poorly dated. Despite evidence for marked variations in climate over the past 2000 years, changes during the 20th century, especially in temperature, may have been unprecedented. The density of data points over western China is currently too low for spatial patterns to be identified, especially in precipitation variation. However, there does seem to have been an increase in warming with altitude over the most recent past.

Alfred Jackson was an enslaved African American born on The Hermitage plantation (outside Nashville, Tennessee) of President Andrew Jackson around 1810, and lived most of his life on the plantation. Staff from The Hermitage wished to better understand and interpret lifeways of those once enslaved on the plantation, but the date of construction of Alfred's Cabin first had to be determined. Was it built when Alfred was enslaved or was a freedman? We extracted nearly 100 core samples from eastern red cedar logs used to construct the cabin. Of these, 39 cores were used to develop a master tree-ring chronology for the cabin. Crossdating of the Alfred's Cabin chronology was accomplished using an eastern red cedar chronology developed by the Lamont-Doherty Earth Observatory Tree-Ring Laboratory at Columbia University in Palisades, New York. The 39 series from Alfred's Cabin resulted in a high-quality master tree-ring chronology, with an average inter-series correlation of 0.66 and an average mean sensit...

We investigate the use of a network of 194 tree-ring chronologies from the “High Asia” region of the Himalaya, Karakoram, Tien Shan, and Tibetan Plateau for the reconstruction of large-scale summer monsoon drought variability over India and the Tibetan Plateau. This experiment was carried out to see if the High Asia tree-ring network contains large-scale information related to the north-south climate gradient over this region, which is known to be associated with the development of the Indian summer monsoon. The climate field chosen for experimental reconstruction was a 735-point grid of self-calibrated Palmer Drought Severity Indics (PDSI) calculated from CRU TS 2.1 gridded temperature and precipitation data. This PDSI grid was reconstructed for the summer monsoon season from a subset of tree-ring chronologies covering the period 1600-1989 using a reduced-space multivariate regression method, and both calibration and verification tests indicate that the reconstructions contain significant hindcast skill in substantial sub-regions of the grid, especially over India. Principal components analysis carried out on the almost 400 years of data over the complete grid indicates that the leading spatial mode or EOF of summer monsoon PDSI expresses a dominant north-south alternation in drought and wetness between India and the Tibetan Plateau, a leading mode also present in the instrumental data. Wavelet analysis of the time series expression of this leading EOF also reveals a very stable 40-60 year pattern of multidecadal variability in it. This time scale of variability appears to be related to slowly changing tropical ocean sea surface temperatures. Together, these results validate the use of a geographically restricted set of tree-ring chronologies over High Asia for the reconstruction of large-scale summer monsoon variability over India and the Tibetan Plateau.

The possible role that tropical Pacific SSTs played in driving the megadroughts over North America during the medieval period is addressed. Fossil coral records from the Palmyra Atoll are used to derive tropical Pacific SSTs for the period from a.d. 1320 to a.d. 1462 and show overall colder conditions as well as extended multidecadal La Niña–like states. The reconstructed SSTs are used to force a 16-member ensemble of atmosphere GCM simulations, each with different initial conditions, with the atmosphere coupled to a mixed layer ocean outside of the tropical Pacific. Model results are verified against North American tree ring reconstructions of the Palmer Drought Severity Index. A singular value decomposition analysis is performed using the soil moisture anomaly simulated by another 16-member ensemble of simulations forced by global observed SSTs for 1856–2004 and tree ring reconstructions of the Palmer Drought Severity Index for the same period. This relationship is used to transfer the modeled medieval soil moisture anomaly (relative to the modern simulation) into a model-estimated Palmer Drought Severity Index. The model-estimated Palmer Drought Severity Index reproduces many aspects of both the interannual and decadal variations of the tree ring reconstructions, in addition to an overall drier climate that is drier than the tree ring records suggest. The model-estimated Palmer Drought Severity Index simulates two previously identified “megadroughts,” a.d. 1360–1400 and a.d. 1430–60, with a realistic spatial pattern and amplitude. In contrast, the model fails to produce a period of more normal conditions in the early fifteenth century that separated these two megadroughts. The dynamical link between tropical SSTs and the North American megadroughts is akin to that operating in modern droughts. The model results are used to argue that the tropical Pacific played an active role in driving the megadroughts. However, the match between simulated and reconstructed hydroclimate is such that it is likely that both the coral-reconstructed SST anomalies contain significant errors and that SST anomalies in other basins also played a role in driving hydroclimate variations over North America during the late medieval period.

We present a detailed analysis of a rare cross-taxa native species phenology dataset (plant flowering, insect first sighting, and amphibian first sighting) from Mohonk Lake, NY. This dataset is highly unusual in North America for its longevity of record, consistency of methodology and location, diversity of species available, and availability of local daily meteorological data. For each phenology series, we examined the flowering and first sighting Julian calendar dates for the existence of temporal trends. Only one of the five animal species (katydid) showed any evidence for a significant trend in first sighting. In contrast, the plant species showed a rich mixture of temporal trends in flowering that could be divided into four classes: woody plant–no trend, woody plant–negative trend, herbaceous plant–negative trend, and herbaceous plant–positive trend. Many of the trends were found to be statistically significant and robust to the method of trend estimation. The data within each of the four plant classes were also pooled as anomalies to provide more complete temporal coverage for tests of trend robustness. The results were strongly consistent with the trends of the individual species within each class and highly significant statistically. We next correlated the flowering and first sighting dates against growing degree-day (GDD) summations for each day of the year to measure the sensitivity of each species to this common form of climatic forcing on phenology. All species showed a significant sensitivity to GDD summations, with peak correlations falling on or near the varying median flowering or first sighting dates. These results were robust whether the GDD analyses were conducted over the complete set of observations for each species (beginning on or after 1928) or over a latter period with a more serially complete set of cross-taxa observations (1970–2002). The GDD correlations indicate significant climate sensitivity in all species, but the different magnitudes of correlation and timing of maximum correlation imply that plant and animal responses to climate changes in the future will not be homogeneous for the tested species at Mohonk Lake. Copyright © 2007 Royal Meteorological Society

Moisture variability across China and Mongolia (hereafter, CM) during 1951–2005 was investigated using the recently developed monthly Palmer Drought Severity Index (PDSI) dataset. In total there are 206 PDSI grid points across CM, based on a 2.5° × 2.5° gridding system. For CM as a whole a significant decreasing trend in mean moisture availability was observed during 1951–2005, but with strong decadal (17.1-year) and interannual (5.0-year, 3.2-year, 2.4–2.8 year) variations. The areas affected by moderate and severe moisture deficit over CM have increased significantly since the mid-1950s. In contrast, there is a significant decreasing trend for areas affected by moderate wetness since the mid-1950s, and no significant trend was found for the areas affected by severe wetness. Ten moisture-related spatial patterns were objectively defined for CM using rotated Empirical Orthogonal Function (REOF) analysis. These patterns are related to distinct geographical areas and are associated with distinct temporal variations. Four of these patterns, in Northeast China (NE), North China (NC), Central China (CC), and East China (EC), generally demonstrate a significant decreasing trend in moisture availability. Two patterns located in western areas of Northwest China (NW) and the Tibetan Plateau (TP) show a significant moisture increase, while four patterns in Mongolia (MN), far western China (FW), South China (SC), and Southwest China (SW) do not have significant moisture trends during 1951–2005. Based on REOF results we propose that CM should be divided into ten coherent moisture divisions. Moisture variations within each division are generally coherent, but may show either similar or contrasting covariability with adjacent divisions.

The world's hydrological cycle is believed to intensify with global warming, yet current climate models have only a limited ability to assess moisture responses at regional scales. Tree-ring records are a valuable source of information for understanding long-term, regional-scale moisture changes, particularly for large regions such as the Tibetan Plateau (TP), where the observational data are short and sparse. Here, we present a new ring-width chronology developed from Qilian Juniper (Sabina przewalskii) wood at two sites on the northeastern TP. This chronology, combined with others from the same region, demonstrates that tree growth anomalies are linked to regional late spring to early summer moisture availability. Although late monsoon season precipitation in the study area decreased during recent decades, tree growth continued to increase due to persistent moisture availability in the early monsoon season. Comparison with global sea surface temperatures (SSTs) indicates that early (late) monsoon season precipitation is closely related to tropical Pacific (Indian Ocean) SSTs, suggesting a possible seasonal shift in the dominant moisture source area for monsoonal precipitation over the northeastern TP. It is further shown that there is a very high degree of coherency regarding low-frequency tree growth anomalies over the northeastern TP during the last six centuries. The most prominent drought epoch occurred during ca. 1450–1500, which may have been caused by a significant decrease in the thermal gradient between the Eurasian continent and the tropical oceans. A persistent tree growth increase since the 1880s is coincident with global warming, suggesting an intensified early monsoon season moisture regime in the study area.